Once-through vapor generator

ABSTRACT

A method and apparatus for separately introducing an auxiliary feedfluid into a shell-and-tube-type vapor generator when the flow of main feedfluid is discontinued. The auxiliary feedfluid provides the vapor for preheating the entering main feedfluid when flow of the latter is resumed thereby reducing temperature differentials between the shell and tube sheet metal and the entering main feedfluid so as to eliminate thermal shock.

u; w m I mte States [151 3,63%? Sprague 1 Jan. W72

[54] ()NCE.THROIUGH VAPOR GENERATUR 3,447,509 6/1969 Sprague ..122/32 [72] Inventor: Theodore S. Sprague, Hudson, Ohio p i E i ch sukalo [73] Assignee: The Babcock of Wilcox Company, New Atmmey J Maguire York, NY.

[57] ABSTRACT 22 F1 d: M 2 1970 l 1 l 6 ar A method and apparatus for separately introducing an auxilia- [21] Appl. No.: 15,483 ry feedfluid into a shell-and-tube-type vapor generator when the flow of main feedfluid is discontinued. The auxiliary feedfluid provides the vapor for preheating the entering main feed- [52] US. Cl ..165/134, 165/140, 165/158, fluid when flow of the latter is resumed thereby reducing 122/32 1m Cl Fzsf 19/00 perature differentials between the shell and tube sheet metal 58 Field ofSearch ..165/134, 140, 158; 122/32 g g ememg mam feedflu'd as m thermal Reterences Cited 5 Claims, 3 Drawing Figures UNITED STATES PATENTS 3,437,077 4/1969 Ammon et al. ..l22/32 M 2g 3 l 2% 3M 2/ |u 31 3 32 32 2 I 9 M Z 42 a I I 23 4 9 a L i 44 A, 43 BY ONCE-THROUGH VAPOR GENERATOR BACKGROUND OF THE INVENTION This invention relates generally to a shell-and-tube-type vapor generator and more particularly to improvements in the arrangement for introducing a vaporizable feedfluid into a once-through vapor generator. The improvements of the present invention are directed to vapor generators of the type disclosed in U.S. Pat. Nos. 3,385,268 and 3,447,509 issued to the assignee of the present invention wherein a heating fluid is directed through the tubes, and the feedfluid is discharged into the shell and flows downwardly through an annular downcomer and then upwardly through a tube bank and is heated and vaporized as it passes over and along the tubes. A portion of this partially heated fluid is withdrawn from the tube bank to mix with and preheat the feedfluid entering the vapor generator shell, thereby eliminating thermal shock of the thick shell and lower tube sheet metal due to the temperature differential between the metal and the entering feedwater. In order to avert costly repairs and to prolong the life of the vapor generator, stringent metal temperature differential limits have been established for the startup and shutdown of the plant. These operational limitations, however, have the disadvantage of extending the outage time thereby imposing an economic loss due to reduction of plant availability. A particularly disadvantageous situation arises when, due to an emergency condition, the flow of feedfluid may be interrupted and the vapor-generator-heating fluid is evaporated and its metal temperature may approach the temperature of the entering heating fluid. Under these circumstances, even though the emergency condition may be quickly corrected, there will be no heated fluid available to preheat the feedfluid entering the generator shell and the plant may be forced into a protracted shutdown while the vapor generator metal temperatures decrease to a point wherein the addition of feedfluid without the benefit of preheating will not cause temperature differentials in excess of the prescribed limits.

SUMMARY OF THE INVENTION In accordance with the present invention an improvement is made on vapor generators of the type disclosed in U.S. Pat. Nos. 3,385,268 and 3,447,509 by providing an arrangement whereby the introduction of main feedfluid into the vapor generator can be resumed without delay following a stoppage in the flow of feedfluid during the operation of the vapor generator. The arrangement includes a plurality of nozzles penetrating the upper end of the shroud to discharge a controlled quantity of auxiliary feedfluid directly into the inner passage of the vapor generator. A typical auxiliary feedfluid system has a maximum flow capacity of approximately 3.5 percent of the full main feedfluid flow. The shroud and heat exchange tubes are relatively thin-walled and able to withstand the temperature differentials resulting from the introduction of nonpreheated auxiliary feedfluid. Following the loss of main feedfluid flow, auxiliary feedfluid is injected directly into the inner passage and vaporizes as it comes into contact with the hot tubes, resulting in a vapor pressure buildup within the generator. Prompt resumption of main feedfluid flow to the vapor generator is made possible through the use of auxiliary feedfluid vapor to preheat the entering main feedfluid. As soon as normal vapor generation is established, the flow of auxiliary feedfluid can be discontinued.

An alternative use can be made of the auxiliary feedfluid system to remove decay heat from the vapor generator and its associated heat source, e.g., a nuclear reactor, during a plant shutdown. A predetermined rate of cooling can be maintained by regulating the quantity of auxiliary feedfluid introduced into the vapor generator and discharging the resulting vapor directly to the condenser.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a sectional elevation view of a once-through vapor generator embodying the invention;

FIG. 2 is a transverse section taken along line 2-2 in FIG. 1.

FIG. 3 is a transverse section taken along line 33 in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates a heat exchanger in the form of a oncethrough vapor generating and superheating unit It) comprising a vertically elongated cylindrical pressure vessel II closed at its opposite ends by an upper head member I2 and a lower head member 13. The vessel 11 is transversely divided by upper and lower tube sheets 14 and I5 respectively. The upper tube sheet 14 is integrally attached to vessel II and upper head member 12 and forms in combination with the upper head member a fluid inlet chamber I6. The lower tube sheet 15 is integrally attached to vessel 11 and lower head member 13 and forms in combination with the lower head member a fluid outlet chamber I7.

A multiplicity of straight tubes 18 arranged to form a tube bank extend vertically between the upper and lower tube sheets M and I5 and penetrate through both tube sheets to interconnect the fluid inlet chamber 16 with the fluid outlet chamber I7. A cylindrically shaped lower shroud member I9 surrounds the tubes 18 and extends upwardly from the upper face of the lower tube sheet I5 and terminating at a plane intermediate the height of vessel II. This lower shroud defines the lower portion of an inner passage or steam-generating riser chamber 20 which contains the lower portion of tubes 13 and cooperates with the vessel II to form the lower portion of a circumscribing annular-shaped outer passage or inlet compartment 21. Openings 22 circumferentially spaced about the lower portion of shroud I9 provide flow communication between the inlet compartment 2I and the riser chamber 20. An adjustable circular segmental plate orifice 23 projects outwardly from the shroud 19 at approximately the level of the top edge of openings 22.

A cylindrically shaped upper shroud member 24 extends upwardly from a plane closely spaced above the upper edge of lower shroud I9 to a plane located below the upper tube sheet 14. This upper shroud 24 forms the upper portion of an inner passage or steam-generating and superheating chamber 25 and being an extension of chamber 20, contains the upper section of tubes 18. The shroud 24 in cooperation with the vessel 11 forms the upper portion of an annular-shaped outer passage or outlet compartment 26. The lower end of compartment 26 is sealed closed by an annular plate 27 welded about its outer edge to the vessel 11 and around its inner edge to the shroud 24. The open space 28 between the top edge of shroud I9 and the bottom plate 27 of shroud 24 is in flow communication with the inlet compartment 21. A number of tube supports 29 are spaced along the length of the bank of tubes I8 within the chambers 20 and 25.

At the upper end of the inlet compartment 21, a plurality of main feedfluid nozzles 30 extend through the wall of vessel II with their respective outlet ends discharging into the inlet compartment 21 near or at the same level as the open space 28 and as shown by the spray pattern at 30A. Connecting pipes 31 join nozzles 30 to a ring-shaped main feedfluid heater 32 which encircles the vessel Ill below the nozzles 30.

Near the upper end of the outlet compartment 26, a plurality of auxiliary feedfluid nozzles 33 extend through the wall of vessel 11 and the upper shroud 24% with their respective outlet ends discharging into the steam-generating and superheating chamber 25 as shown by the spray pattern at 33A. Connecting pipes 34 join nozzles 33 to a ring-shaped auxiliary feedfluid header 35 which encircles the vessel II below the nozzles 33.

The upper head member 12 is provided with an inlet connection 36 for admitting heating fluid to chamber I6 while lower head member I3 is provided with an outlet connection 37 for discharging the heating fluid from chamber I7. The vessel I1 includes outlet connections 38 for delivering the superheated vapor to the point of use, and manway 39 and inspection ports 40 and 41 which provide physical and visual access to the interior of the vessel. Also included are fluid level sensing connections 42, vent connection 43 and drain connections 44, The upper and lower heads 12 and 13 are provided with manways 45 and 46 and inspection ports 47 and 48 respectively and lower head 13 also includes a drain connection 49.

FIG. 2 illustrates a transverse section of the once-through vapor-generating and superheating unit taken at section 2-2 of FIG. 1, i.e., at the auxiliary feed inlet to the unit. The auxiliary feedfluid nozzles 33 are shown spaced circumferentially about the vessel 11 and extending through the vessel wall, across the outlet compartment 26 and through the upper shroud member 24 to discharge directly into the upper portion of the inner passage of the generating and superheating chamber 25 as shown at 33A. An auxiliary feedfluid header 35, made up of two arcuate sections joined by a flanged connection 50, supplies fluid through the connecting pipes 34 to the nozzles 33 for discharge over the outside of tubes 18.

FIG. 3 illustrates a transverse section of the once-through vapor-generating and superheating unit 10 taken at section 3-3 of FIG. 1, i.e., at the main feedfluid inlet to the unit and including the multiple main feedfluid nozzles 30, only five of which are actually shown, spaced circumferentially about the vessel 11 and extending through the vessel wall to discharge directly downward into the inlet compartment 21. A main feedfluid header 32, made up of two separate arcuate sections, supplies fluid through the connecting pipes 31 to the nozzles 30 for discharge into compartment 21. The lower shroud member 19 defines the outer periphery of the lower portion of the inner passage or riser chamber 20 which houses the lower length section of tubes 18.

During normal operation of the vapor generator, primary coolant received from a pressurized water reactor or a similar source, not shown, is supplied to the upper chamber 16 through the inlet connection 36. The primary coolant gives up heat to a secondary fluid during passage through the tubes 18 of vapor generator MD and thus will hereinafter be referred to as the heating fluid. From chamber 16, the heating fluid flows downwardly through the tubes 18 into the lower chamber 17 and is discharged from the vapor generator through the outlet connection 37. The feedfluid supplied to the header 32 from whence it is discharged through the nozzles 34) into the upper end of the inlet compartment 21 of the vapor generator. The feedfluid flows downwardly through the inlet compartment 21 and past the adjustable orifice 23 and through the shroud openings 22 into the riser chamber 20. The main feedfluid enters the riser chamber 20 at substantially saturation temperature and vapor generation commences immediately. It flows upwardly about the tubes in counterflow and indirect heattransfer relationship with the heating fluid flowing within the tubes 18.

As the main feedfluid flows upwardly through the riser chamber 24), vapor is generated ranging from zero quality at the lower tube sheet to substantially lOO-percent quality adjacent the upper end of the lower shroud 19. A portion of the main feedfluid in the form of vapor at substantially I00- percent quality is withdrawn from the top of shroud l9 and passed through the open space 28 to mix with and heat the main feedfluid being discharged from the nozzles 30. As this vapor mixes with the incoming feedfluid, it condenses resulting in a slight reduction in pressure which provides an aspirating effect causing the withdrawal of vapor from within the chamber into the inlet compartment 21. The withdrawn vapor gives up its latent heat of vaporization to the incoming feedfluid with the mixture being heated substantially to saturation temperature. That portion of vapor which has not been withdrawn is passed upwardly through the superheating chamber 25 and is superheated before it reverses direction about the upper shroud 24. it then flows downwardly through the outlet compartment 26 between the upper shroud and the shell and finally exits from the unit through the vapor outlet connections.

In accordance with the present invention, whenever there is a complete stoppage of the main feedfluid supply to the vapor generator, the nuclear reactor is automatically shutdown while the primary coolant or heating fluid continues to pass through the reactor and through the vapor generator at a selected rate of flow. The auxiliary feedfluid system becomes activated substantially simultaneously with the loss of the main feedfluid supply. Auxiliary fluid is supplied to the header 35 and injected through the nozzles 33 directly into the upper portion of the inner passage or superheating chamber 25 some of which vaporizes as it comes into contact with the heated tubes 18 resulting in a vapor pressure buildup within the generator. Auxiliary feedfluid continues to flow in at a selected rate of flow so as to maintain a preset minimum water level. Decay heat from the reactor is removed by continuing the indirect heat exchange between the primary coolant and the auxiliary feedfluid and discharging the resulting steam directly to the condenser.

1n the event that a hot restart is contemplated, introduction of main feedfluid to the generator may be resumed as soon as the supply becomes available. The possibility of thermally shocking the hot metal of the vessel 11 and the tube sheet 15, due to temperature differentials created by the relatively cool incoming main feedfluid, is eliminated by withdrawing the vapor portion of the auxiliary fluid through the open space 28 and mixing it directly with the main feedfluid being discharged by the nozzles 30 thereby preheating the latter fluid before any substantial contact is made with the heated generator metal. The flow of auxiliary feedfluid is continued until normal main feedfluid vapor generation is resumed at which time the flow of auxiliary fluid is discontinued.

In the event that the plant is scheduled for a shutdown, the rate of cooling for the nuclear reactor and generating equipment may be predetermined by regulating the quantity of auxiliary feedfluid flowing into the vapor generator.

in a typical nuclear-powered steam generator, the auxiliary feedwater system has a flow range from 0 to approximately 3.5 percent of full load main feedwater flow and is capable of removing up to 5 percent of reactor power, assuming a feedwater inlet temperature of F, and generation of saturated steam at full load steam pressure conditions.

What is claimed is:

l. A heat exchanger comprising:

a pressure vessel,

a plurality of tubes extending through said vessel,

a shroud means surrounding the tubes and cooperating with the vessel to form inner and outer passages, said outer passage being separated into inlet and outlet compartments, each of said compartments flow-communicating with the inner passage,

means for directing a heating fluid through the tubes,

means for introducing and serially directing a first feedfluid in through the inlet compartment, through the inner passage in indirect heat exchange relation with the heating fluid, and out through the outlet compartment,

means for withdrawing a portion of the partially heated first feedfluid from the inner passage and directing it in mixing relation with the first feedfluid entering the inlet compartment, and

separate means for directly introducing a second feedfluid into said inner passage when flow of said first fluid is discontinued.

2. A heat exchanger according to claim 1 wherein the means for introducing the first feedfluid includes a plurality of nozzles discharging into said inlet compartment.

3. A heat exchanger according to claim 1 wherein the shroud means comprises separate upper and lower shroud members, said upper and lower members cooperating with the vessel to form outlet and inlet compartments respectively.

4. A heat exchanger according to claim 3 wherein the separate means for introducing the second feedfluid includes a plurality of nozzles extending through the upper shroud member for discharging into the inner passage.

5. A heat exchanger according to claim 3 wherein the means for withdrawing a portion of the partially heated fluid includes at least one opening between adjacent ends of said upper and lower shroud members for providing flow communication between the inner passage and the inlet compart- 5 ment. 

1. A heat exchanger comprising: a pressure vessel, a plurality of tubes extending through said vessel, a shroud means surrounding the tubes and cooperating with the vessel to form inner and outer passages, said outer passage being separated into inlet and outlet compartments, each of said compartments flow-communicating with the inner passage, means for directing a heating fluid through the tubes, means for introducing and serially directing a first feedfluid in through the inlet compartment, through the inner passage in indirect heat exchange relation with the heating fluid, and out through the outlet compartment, means for withdrawing a portion of the partially heated first feedfluid from the inner passage and directing it in mixing relation with the first feedfluid entering the inlet compartment, and separate means for directly introducing a second feedfluid into said inner passage when flow of said first fluid is discontinued.
 2. A heat exchanger according to claim 1 wherein the means for introducing the first feedfluid includes a plurality of nozzles discharging into said inlet compartment.
 3. A heat exchanger according to claim 1 wherein the shroud means comprises separate upper and lower shroud members, said upper and lower members cooperating with the vessel to form outlet and inlet compartments respectively.
 4. A heat exchanger according to claim 3 wherein the separate means for introducing the second feedfluid includes a plurality of nozzles extending through the upper shroud member for discharging into the inner passage.
 5. A heat exchanger according to claim 3 wherein the means for withdrawing a portion of the partially heated fluid includes at least one opening between adjacent ends of said upper and lower shroud members for providing flow communication between the inner passage and the inlet compartment. 